1. Field of the Invention
The present invention relates to a drilling tool.
2. Description of the Related Art
In recent years, in the case where a workpiece having a complicated shape such as an automobile engine member, etc. is subjected to a drilling machining, chips caused in the machining have been strongly required to be more minute from the viewpoint of improving the chip controllability.
In order to satisfy this requirement, a nicked drill has been known in which a nick for dividing chips is provided by cutting a part of the drill tip end along a flank face (for example, refer to Japanese Patent Laid-Open No. 2007-50477).
This nicked drill is configured so that each of a plurality of cutting edges has a plurality of concave groove-like nicks vertical to the cutting edge. These nicks are arranged in the cutting edges so that the rotation trajectories of the nicks in the circumferential direction around the axis line are different with respect to the cutting edges adjacent to one another, and thereby the nicks divide chips in the width direction within a short range of each nick.
However, in the case of the invention of Japanese Patent Laid-Open No. 2007-50477, nicks provided from the cutting edge along the flank face causes a possibility where a cutting edge at the drill tip end may be grinded together with the nick periphery to thereby eliminate the existence of the nick. Thus, a disadvantage has been caused where the nick function must be maintained by forming, prior to the use of the drill, the nicks of the drill repeatedly by a grinding machining. If the nicks are formed in an extended manner in the vertical direction to the cutting edge in order to avoid the repeated machining of the drill for forming the nicks as described above, a disadvantage of deteriorated cutting edge strength is caused.
Furthermore, the nicks arranged so that a rotational trajectory of the nicks are dislocated with respect to the cutting edges adjacent to one another in the circumferential direction around the axis line. Thus, the nicks are prevented from having a freely-set groove width, position, or number, and thus causing limitation on the width of chips that can be divided.
Furthermore, the invention of Japanese Patent Laid-Open No. 2007-50477 is also disadvantageous in that the invention of Japanese Patent Laid-Open No. 2007-50477 cannot be applied to a one-blade rotation tool because of the configuration where the chip of the residual stock after forming of a machined hole by the nicks cut by a preceding cutting edge in the drill rotation direction is cut by a subsequent cutting edge having a dislocated nick rotation trajectory.
Thus, in order to solve the above disadvantages, the applicant of this application has suggested, in Japanese Patent Laid-Open No. 2009-202288, a method of providing more minute chips according to which a rake face is formed to have a step-like shape having at least one step section so that the cutting edge is divided in the diameter direction of the tool body. The drilling tool as described above does not need to perform the machining for forming nicks that is repeatedly performed in order to polish the cutting edge. This drilling tool can change the number of the step sections or the width in the diameter direction for example to adjust the lengths of the respective divided cutting edges in the diameter direction. Thus, chips can be segmented to have a desired width. This drilling tool also can be applied to a one-blade gun drill or a reamer for example.
However, when the rake face is formed to have a step-like shape as described above, many parts are required to form grooves for discharging chips, which causes a concern that the tool may have a declined rigidity when the tool is used for a deep hole machining drill.